Atleast one blue marble

A bag of 10 marbles contains 3 red marbles and 7 blue marbles. If two marbles are selected at random, what is the probability that at least one marble is blue?

Explain ans in detail.

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gmat_dest: Master | Next Rank: 500 Posts; Posts: 186; Joined: Fri Jan 16, 2009 4:57 am; Thanked: 7 times; GMAT Score:720

by gmat_dest » Mon Aug 03, 2009 9:08 am

Probability of at least one blue = 1 - Probability of all red

Probability of both red = 3/10 * 2/9 = 1/15

So, probability of at least one blue = 14/15.

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maihuna: Legendary Member; Posts: 1578; Joined: Sun Dec 28, 2008 1:49 am; Thanked: 82 times; Followed by:9 members; GMAT Score:720

by maihuna » Mon Aug 03, 2009 9:11 am

No marble is blue means both are red: = 3/10*2/9 = 1/15

So at least one blue = 1-1/15 = 14/15

Charged up again to beat the beast

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